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I have a Sharan 1998 model that comes with only a key a blue button that ha the volks logo drawn on ...

I have a Sharan 1998 model that comes with only a key a blue button that ha the volks logo drawn on it. I can only lock the doors centrally from the inside. Can someone pls tell me what the blue button is for and how to centrally lock the car from the inside to prevent harassment or attacks from hoodlums